A seismic survey represents an attempt to image or map the subsurface of the earth by sending sound energy down into the ground and recording the “echoes” that return from the rock layers below. The source of the down-going sound energy might come, for example, from explosions or seismic vibrators on land, or air guns in marine environments. During a seismic survey, the energy source is placed at various locations near the surface of the earth above a geologic structure of interest. Each time the source is activated, it generates a seismic signal that travels downward through the earth, is reflected, and, upon its return, is recorded at a great many locations on the surface. Multiple source/recording combinations are then combined to create a near continuous profile of the subsurface that can extend for many miles. In a two-dimensional (2D) seismic survey, the recording locations are generally laid out along a single line, whereas in a three dimensional (3D) survey the recording locations are distributed across the surface in a grid pattern. In simplest terms, a 2D seismic line can be thought of as giving a cross sectional picture (vertical slice) of the earth layers as they exist directly beneath the recording locations. A 3D survey produces a data “cube” or volume that is, at least conceptually, a 3D picture of the subsurface that lies beneath the survey area. In reality, though, both 2D and 3D surveys interrogate some volume of earth lying beneath the area covered by the survey.
A seismic survey is composed of a very large number of individual seismic recordings or traces. In a typical 2D survey, there will usually be several tens of thousands of traces, whereas in a 3D survey the number of individual traces may run into the multiple millions of traces. Chapter 1, pages 9-89, of Seismic Data Processing by Ozdogan Yilmaz, Society of Exploration Geophysicists, 1987, contains general information relating to conventional 2D processing and that disclosure is incorporated herein by reference. General background information pertaining to 3D data acquisition and processing may be found in Chapter 6, pages 384-427, of Yilmaz, the disclosure of which is also incorporated herein by reference.
A seismic trace is a digital recording of the acoustic energy reflecting from inhomogeneities or discontinuities in the subsurface, a partial reflection occurring each time there is a change in the elastic properties of the subsurface materials. The digital samples are usually acquired at 0.002 second (2 millisecond or “ms”) intervals, although 4 millisecond and 1 millisecond sampling intervals are also common. Each discrete sample in a conventional digital seismic trace is associated with a travel time, and in the case of reflected energy, a two-way travel time from the source to the reflector and back to the surface again, assuming, of course, that the source and receiver are both located on the surface. Many variations of the conventional source-receiver arrangement are used in practice, e.g. VSP (vertical seismic profiles) surveys, ocean bottom surveys, etc. Further, the surface location of every trace in a seismic survey is carefully tracked and is generally made a part of the trace itself (as part of the trace header information). This allows the seismic information contained within the traces to be later correlated with specific surface and subsurface locations, thereby providing a means for posting and contouring seismic data—and attributes extracted therefrom—on a map (i.e., “mapping”).
The data in a 3D survey are amenable to viewing in a number of different ways. First, horizontal “constant time slices” may be taken extracted from a stacked or unstacked seismic volume by collecting all of the digital samples that occur at the same travel time. This operation results in a horizontal 2D plane of seismic data. By animating a series of 2D planes it is possible for the interpreter to pan through the volume, giving the impression that successive layers are being stripped away so that the information that lies underneath may be observed. Similarly, a vertical plane of seismic data may be taken at an arbitrary azimuth through the volume by collecting and displaying the seismic traces that lie along a particular line. This operation, in effect, extracts an individual 2D seismic line from within the 3D data volume. It should also be noted that a 3D dataset can be thought of as being made up of a 5D data set that has been reduced in dimensionality by stacking it into a 3D image. The dimensions are typically time (or depth “z”), “x” (e.g., North-South), “y” (e.g., East-West), source-receiver offset in the x direction, and source-receiver offset in the y direction. While the examples here may focus on the 2D and 3D cases, the extension of the process to four or five dimensions is straightforward.
Seismic data that have been properly acquired and processed can provide a wealth of information to the explorationist, one of the individuals within an oil company whose job it is to locate potential drilling sites. For example, a seismic profile gives the explorationist a broad view of the subsurface structure of the rock layers and often reveals important features associated with the entrapment and storage of hydrocarbons such as faults, folds, anticlines, unconformities, and sub-surface salt domes and reefs, among many others. During the computer processing of seismic data, estimates of subsurface rock velocities are routinely generated and near surface inhomogeneities are detected and displayed. In some cases, seismic data can be used to directly estimate rock porosity, water saturation, and hydrocarbon content. Less obviously, seismic waveform attributes such as phase, peak amplitude, peak-to-trough ratio, and a host of others, can often be empirically correlated with known hydrocarbon occurrences and that correlation applied to seismic data collected over new exploration targets.
Many variations of the conventional source-receiver arrangement are used in practice, e.g. VSP (vertical seismic profile) surveys, ocean bottom surveys, etc.
For all of its usefulness, seismic data suffer from a variety of problems and shortcomings. In more particular, in areas of complex geology the images produced by even the best seismic survey may fail to accurately image important details of the subsurface.
As a specific example, it is well known that the location of the salt-sediment interface is of great importance to people involved in oil exploration and production in the Gulf of Mexico, and many other regions. In such regions, reservoirs may be formed by the truncation of dipping sediments against the flank of a salt done. As a consequence, being able to recognize and determine the location of such a truncation in a seismic data set is of particular importance and would be used to position boreholes that are to be drilled for exploration and production purposes, estimate reservoir reserves, etc.
Unfortunately, in many cases the target salt flank cannot be readily seen on images that have been formed from surface seismic data (i.e., a from data collected during a conventional surface seismic survey). Further, in those cases where a salt-sediment interface is believed to be visible, there still may be considerable uncertainty in its precise location (horizontally and in depth) due at least in part to the large velocity contrast between salt and sediments. Additionally, differences in anisotropic behavior (the effect of the velocity of propagation of seismic energy being dependent on the direction of propagation) between salt and sediments can result in the apparent location of the interface being offset from its true position in the subsurface.
As a consequence, when boreholes are drilled they are often used to obtain a more precise understanding of the location of the salt-sediment interface. Conventionally, this might be done by conducting a salt proximity survey in the well. Those of ordinary skill in the art will understand that a salt proximity survey is a refraction survey that is conducted by exciting a surface source at a single location that has been chosen to be proximate to the top of a salt dome, and recording the shot at multiple receiver position within the well. Once the velocities corresponding to the first arrivals have been determined (the velocity in salt being much faster than the velocity in the surrounding sediment), ray-tracing might then be used to obtain a profile of the salt flanks relative to the borehole.
While salt proximity surveys provide useful information about the subsurface, that information is limited in its usefulness away from the borehole. For example, the traditional salt proximity provides reasonably good estimates of the salt-sediment interface along a line between the source location and the borehole, but it may provide a poorly constrained solution elsewhere in 3D space, especially for rugous surfaces.
Heretofore, as is well known in the seismic processing and seismic interpretation arts, there has been a need for a method of obtaining better estimates of the location of a salt dome flank within the subsurface. Accordingly, it should now be recognized, as was recognized by the present inventor, that there exists, and has existed for some time, a very real need for a method of seismic data processing that would address and solve the above-described problems.
Before proceeding to a description of the present invention, however, it should be noted and remembered that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or preferred embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.